Conventionally, in an air-conditioning apparatus, such as a multi-air-conditioning apparatus for a building, refrigerant is circulated, for example, between an outdoor unit installed outside the building and serving as a heat source unit, and an indoor unit installed in a room of the building. The refrigerant transfers or receives heat, so that an air-conditioned space is cooled or heated with heated or cooled air. A refrigerant often used in such an air-conditioning apparatus is, for example, a hydrofluorocarbon-based (HFC-based) refrigerant. Air-conditioning apparatuses using a natural refrigerant, such as carbon dioxide (CO2), are also proposed.
In an air-conditioning apparatus called a chiller, a heat source unit installed outside the building generates cooling energy or heating energy. Then, a heat exchanger installed inside the outdoor unit heats or cools a heat medium, such as water or antifreeze. The heated or cooled heat medium is conveyed, for example, to a fan coil unit or panel heater serving as an indoor unit, which performs cooling or heating (see, e.g., Patent Literature 1).
Also, there is an air-conditioning apparatus called a waste heat recovery chiller in which four water pipes are connected between a heat source unit and an indoor unit, so that cooled water and heated water are supplied at the same time. This allows free selection of either cooling or heating in the indoor unit (see, e.g., Patent Literature 2).
Also, there is an air-conditioning apparatus with a configuration in which a heat exchanger for a primary refrigerant and a secondary refrigerant is installed near each indoor unit, and the secondary refrigerant is conveyed to the indoor unit (see, e.g., Patent Literature 3).
Also, there is an air-conditioning apparatus with a configuration in which an outdoor unit and a branching unit including heat exchangers are connected to each other by two pipes, and a secondary refrigerant is conveyed to an indoor unit (see, e.g., Patent Literature 4).
Also, there is an air-conditioning apparatus, such as a multi-air-conditioning apparatus for a building, which circulates heat-source-side refrigerant from an outdoor unit to a relay unit, and also circulates a heat medium, such as water, from the relay unit to indoor units. The air-conditioning apparatus thus reduces conveying power of the heat medium, such as water, while circulating the heat medium through the indoor units (see, e.g., Patent Literature 5).